In continuous casting of steel, molten steel placed in a tundish is poured into a mold for continuous casting via an immersion nozzle connected to the tundish bottom. In this case, the molten steel flow discharged from a spout of the immersion nozzle to inside a mold is accompanied with non-metallic inclusions (mainly, deoxidization products such as alumina) and bubbles of inert gas (inert gas injected to prevent nozzle clogging caused by adhesion and accretion of alumina and the like) injected from an inner wall surface of an upper nozzle. However, when the non-metallic inclusions and bubbles are entrapped in a solidification shell, product defects (defects originating in inclusions and bubbles) occur. Furthermore, a mold flux (mold powder) is entrained in a molten steel upward flow reaching a meniscus and also becomes trapped in the solidification shell, resulting in product defects.
It has been a conventional practice to apply magnetic fields to the molten steel flow in a mold to control the flow of the molten steel through electromagnetic force of the magnetic fields in order to prevent non-metallic inclusions, mold flux, and bubbles in molten steel from becoming entrapped in a solidification shell and forming product defects. Many proposals have been made regarding this technique.
For example, patent document 1 discloses a method for controlling a molten steel flow by DC magnetic fields respectively applied to a pair of upper magnetic poles and a pair of lower magnetic poles that face each other with a mold long-side portion therebetween. According to this method, a molten flow is divided into an upward flow and a downward flow after discharged from a spout of an immersion nozzle, the downward flow is braked with a DC magnetic field in the lower portion, and the upward flow is braked with a DC magnetic field in the upper portion so as to prevent the non-metallic inclusions and mold flux accompanying the molten steel flow from becoming trapped in a solidification shell.
Patent document 2 discloses a method with which a pair of upper magnetic poles and a pair of lower magnetic poles are provided to face each other with a mold long side portion therebetween as in patent document 1 and magnetic fields are applied using these poles where (1) a DC magnetic field and an AC magnetic field are simultaneously applied to at least the lower magnetic poles or (2) a DC magnetic field and an AC magnetic field are simultaneously applied to the upper magnetic poles and a DC magnetic field is applied to the lower magnetic poles. According to this method, the molten steel flow is braked with the DC magnetic field as in patent document 1 while the molten steel is stirred with the AC magnetic field so as to achieve an effect of cleaning non-metallic inclusions and the like at the solidification shell interface.
Patent document 3 discloses a method for braking a molten steel flow by using DC magnetic fields respectively applied to a pair of upper magnetic poles and a pair of lower magnetic poles facing each other with a mold long side portion therebetween and by optionally simultaneously applying an AC magnetic field to the upper magnetic poles, in which the strengths of the DC magnetic fields, the ratio of the strength of the DC magnetic field of the upper electrodes to that of the lower electrodes, and the strength of (the upper AC magnetic field, optionally) are controlled within particular numeric ranges. Patent document 4 discloses a technique of producing a continuously cast slab having a graded composition in which the concentration of a particular solute element is higher in a surface layer portion than in the interior of the slab. According to this technique, a DC magnetic field is applied in a direction intersecting the thickness of the slab by using magnetic poles disposed at two stages, i.e., upper and lower stages, so as to increase the concentration of the solute element in the molten steel in an upper pool while a shifting AC magnetic field is simultaneously applied with the DC magnetic field during magnetic field application in an upper portion. However, according to the technique disclosed in patent document 4, the shifting AC magnetic field is applied to induce a flow that eliminates local nonuniformity of the solute concentration.